That's a pretty good analysis! I do tend to wonder why these spiders are doing this though. It must be to mate with these fertile human females. I wonder why though? Perhaps they do not have any females of their own species to mate with, so they must resort to copulating with other species' females, mainly humans. And judging by the size of that low hanging ballsack, there's a lot of sperm there. I think any human female who lets it ejaculate inside them will end up pregnant with its spawn for sure!
It's not wasting its seed. This is some kind of alien spider. It's sperm is probably compatible with human eggs which is why it decided to mate with her in the first place. She'll likely give birth to dozens of spider babies in a few months.